Tritium Science and Engineering for Energy and Environment

Staff：　
HATANO Yuji　HARA Masanori

　Tritium is used as fuel of future fusion reactors. Hence, research and development of safe tritium handling techniques are indispensable for realization of fusion energy. Those techniques are also required for decommissioning of fusion reactors and other nuclear systems. Tritium can be used as a tracer of hydrogen isotopes to study behavior of hydrogen in materials and chemical systems. From these viewpoints, we are studying (1) tritium measurement techniques for gas, liquid and solid phases, (2) interaction of tritium with materials and its control, (3) visualization of hydrogen with tritium, and (4) tritium storage, supply and recovery techniques, as described below in detail.

1. Tritium measurement for gas, liquid and solid phases

　Tritium detection with conventional radiation detectors is difficult, because the maximum energy of beta-rays emitted by tritium decays is only 18.6 keV. Majority of beta-rays are absorbed by sample itself and air before reaching sensitive area of detectors. Therefore, we have developed several unique techniques for tritium measurements as follows. These tritium measurement techniques have attracted tritium researcher in the world.

(1)Beta-ray induced X-ray spectrometry (BIXS): Detection of photons induced by tritium beta-rays

　The interaction of beta-rays from tritium with matters generates energetic photons (characteristics and bremsstrahlung X-rays). The escape depth of X-rays is far larger than that of bera-rays. Hence, this method allows to non-destructive detection of tritium in a deep region of liquid and solid materials. Analysis of energy spectrum of X-rays gives depth profiles of tritium. This method also allows measurements of tritium concentrations in gas and liquid phases from outside of containers through a window made of low atomic number materials such as Be and C.

　The kinetic energy of tritium beta-rays is finally converted into heat in matters. The amount of heat generated by 1 GBq (1×10⁹ decays per second) corresponds to 1 microwatt. We have developed a calorimeter sufficiently sensitive to detect this small amount of heat generated by tritium. This technique provides an absolute value of tritium radioactivity with high accuracy.

2.Interaction of tritium with materials and its control

　Among all the elements, hydrogen atom has the smallest size. Hence, hydrogen isotopes are dissolved and transported in a crystal lattice of solid materials rather easily and trapped at lattice imperfections such as vacancies and second-phase precipitates. Permeation through metals and ceramics is possible at elevated temperatures. Reduction of tritium accumulation and permeation in materials is a key issue in future fusion nuclear reactors to minimize amounts of contaminated wastes and tritium leakage into the work environment. We have examined mechanisms underlying tritium trapping and diffusion in various materials using tritium measurement techniques developed in our center to control tritium accumulation and permeation. Development of new alloys and permeation barrier coatings has also been performed. These studies have been the major field of our domestic and international collaborations.

3. Visualization of hydrogen with tritium

　Many metals and alloys including steels become brittle after absorption of hydrogen from the environment via chemical and electrochemical reactions with water and other hydrogen-containing molecules. This phenomenon is known as "hydrogen embrittlement". The embrittlement results in reduced life time of mechanical parts or, in the worst case, fatal breaking of machines. Visualization of hydrogen location in metals and alloys is important for better understanding of mechanisms underlying hydrogen embrittlement. However, detection of hydrogen in solids is a difficult task because hydrogen atom has only one electron and does not emit signals such as characteristic X-ray, Auger electron, etc. The tracer technique using tritium can solve this problem due to beta-ray emission of tritium. In our center, visualization of hydrogen locations with tritium is possible in different scales from sub-micron to millimeter using different techniques of radiation imaging.

4. Tritium storage, supply and recovery

　In fusion reactors, tritium is supplied to burning plasma as fuel, and unburnt tritium and impurities are pumped out. Tritium in exhausted gas must be separated from impurities and recovered at high efficiency to be used again. In this circulating flow of tritium, various physicochemical processes will be used. Hydrogen permeation membrane can be used for removal of impurities from tritium gas. For separation of hydrogen isotopes, cryogenic distillation and gas chromatography techniques have been proposed. Tritium will be stored in metal getters (hydrogen absorption alloys). The various types of metal getters have been developed in our center together with techniques for the non-destructive measurements of tritium concentration in getters.